During the development of our fibrinolytic assay system we noted an enhancement of fibrinolytic activity in plasma which could not be accounted for by any known fibrinolytic factor. Using heparin affinity chromatography, with a 0.5M NaCl elution and an immuno affinity column we have been able to partially purify this factor. The immuno affinity column uses immobilized MCab-3, a monoclonal antibody against a plasma eluate from a heparin affinity column. We have demonstrated the enhancing activity of the protein in the enzyme linked fibrinolytic assay (ELFA), a clot lysis assay with added t-PA, and a bio immunoassay using a transferable solid phase enzyme labelled fibrin. Using the bio immunoassay we have estimated plasma levels to be approximately 4.5 units. Plasma protein levels for the cofactor appear to be less than 50ug/ml. HPLC gel permeation chromatography reveals a MCab-3 binding protein of 78KD. The protein could not be activated in the presence of high concentrations of extrinsic fibrinolytic activators, precluding the possibility of plasminogen being responsible for the activity. The protein did not activate plasminogen and had no measurable fibrinolytic activity. Immunodiffusion analysis of the intact protein revealed non identity with vitronectin or histidine rich glycoprotein, other heparin binding plasma proteins implicated in fibrinolysis modulation. We propose to purify the protein to homogeneity and positively identify it by N-terminal sequence analysis. We propose raising a polyclonal antibody against the protein to use in the development of immunoassays for the measurement of the protein in plasma.